A. Field of the Invention
The present invention relates to a colour television decoder. Such a decoder may be used for decoding PAL and NTSC colour television signals to provide separate colour components, i.e. either red green and blue (RGB) or luminance (Y) and R-Y (V) and B-Y (U) colour difference signals, for the purpose of direct display of a television picture or for the purpose of processing the decoded signal, possibly before re-encoding into a similar or different system such as PAL, NTSC, SECAM, C-MAC or other format.
B. Description of the Prior Art
The colour information in the PAL or NTSC system is transmitted as a modulation of a subcarrier signal added to the luminance signal. The modulated colour subcarrier is herein referred to as the chrominance signal.
In a conventional PAL or NTSC decoder, the chrominance signal is removed from the input signal to provide luminance by the use of a notch (band-stop) filter designed to suppress the colour subcarrier frequency and also reduce the amplitude of the sidebands generated as a result of its modulation. In practice, the design of this band-stop filter is a poor compromise between adequately suppressing the unwanted chrominance and undesirably suppressing the wanted luminance information at frequencies near to the subcarrier. Inadequate suppression of the chrominance signal results in a fine dot pattern on the displayed luminance signal commonly known as cross-luminance.
The chrominance information in such a conventional decoder is separated from the input signal by bandpass filter designed to select the colour subcarrier and its associated side-bands but reject as far as possible the luminance signal. Again the compromise is unsatisfactory and much unwanted luminance information passes into the chrominance path where it is decoded to produce spurious colour effects commonly known as cross-colour.
A commonly used technique to reduce cross-colour in a PAL decoder is known as delay line decoding. In this type of decoder, the chrominance signal on one line is combined with the chrominance from the previous line which has been delayed by a period of one line plus or minus a period of one quarter of a subcarrier cycle. This technique reduces cross-colour but because of the line averaging also reduces vertical colour resolution. In addition, because of the offset of one quarter of a cycle of subcarrier, horizontal colour resolution is also reduced.
In a comb-filter decoder, cross-colour may be reduced and loss of resolution in vertical luminance is also improved. The comb filter decoder however produces unsatisfactory decoding where the picture contains significant colour changes from one television line to the next. The colour subcarrier is then not adequately suppressed in the luminance output and the vertical resolution of the chrominance outputs is impaired.
Adaptive decoders of the type described in U.K. Patent Application No. 8501305 Ser. No. 2153624A attempt to overcome these limitations by changing the response of the decoder from a comb filter to one incorporating a notch filter if a change in colour from one line to the next exceeds a fixed threshold. This technique improves the decoder performance where distinct colour changes occur as in some electronically generated pictures, but frequently fails to operate adequately on pictures containing more random changes in luminance information or where a more gradual change in picture colour occurs in a scene.